1869.] On a Ternary Geological Classification. 363 



the littoral and the pelagic) which is generally preserved for our 

 observation, as it is only on rare occasions that we meet with the 

 actual margins of geological formations. 



But not only does a natural group represent special regions 

 of the sea-bed, but also, we may suppose, three historical periods of 

 formation, making up in the aggregate a geological cycle ; the first 

 of movement, the second of quiescence, and the third of movement 

 again, terminating in those great oscillations, accompanied by 

 denudation, which brought the cycle to a close, and produced dis- 

 cordant stratification. We have already seen that the formation of 

 calcareous matter depends mainly on the absence of muddy or 

 sandy sediment in the waters of the sea. This state of things 

 would naturally attain predominance after the close of a series of 

 vertical movements, accompanied by a maximum of subsidence of 

 the land. The prevalence of sediment would be brought about 

 during periods of disturbance, accompanied by a maximum of ele- 

 vation of the land. Thus the three stages of a natural cycle going 

 to form a geological group may be thus expressed : — 



(Lower stage, representing prevalence of land with movement, 

 producing chiefly sedimeutarj' strata. 

 Middle stage, representing prevalence of sea with quiescence, 

 producing chiefly calcareous strata. 

 Upper stage, representing prevalence of land with movement, 

 producing chiefly sedimentary strata. 



On the principles here stated it is easy to account for several 

 phenomena of frequent occurrence amongst the formations of our 

 globe. We can account for these interstratifications of calcareous 

 and sedimentary strata, called "passage-beds," of which we have 

 good examples, for instance, at the base of the Yoredale series of 

 England, by supposing the alternate predominance of muddy and 

 clear water in the sea at the margin of the pelagic and littoral 

 regions. We can also account for the fact that a natural group of 

 strata is rarely if ever introduced by a series of limestones, but 

 generally by coarse sedimentary strata, often conglomerates. I 

 question if a true natural group is ever represented by limestones in 

 its lowest beds, and if this should happen to be the case it may be 

 concluded that it is exceptional and due to the local absence of a 

 lower member. A natural group is also seldom terminated by a 

 calcareous stratum, and when this is the case (as, for instance, the 

 chalk of England) it is owing to the local absence of an upper 

 sedimentary member. I am not, however, prepared to say that this 

 is invariably the case, as the Upper Silurian group of the United 

 States seems to ofier an illustration in an opposite direction.* 



* It is questionable, however, whether this is really an exception, as the Oris- 

 kany sandstone might be assumed without much hesitation to be the upper sedi- 

 mentary member of the Silurian ; and the " Cauda g:dli grit " the lower member 

 of the Devonian series. — See ' Siliiria,' 4th edit., pp. 43(3-7. 



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